Optical fiber element having a permanent protective coating with a shore D hardness value of 65 or more

ABSTRACT

An optical fiber element includes an optical fiber having a numerical aperture ranging from 0.08 to 0.34 and a protective coating affixed to the outer surface of the optical fiber. The protective coating has a Shore D [hardnees] hardness value of 65 or more and remains on the optical fiber during connectorization so that the fiber is neither damaged by the blades of a stripping tool nor subjected to chemical or physical attack.

FIELD OF THE INVENTION

The present invention relates to an optical fiber element and, moreparticularly, to an optical fiber element comprising an optical fiberhaving a protective coating affixed to the outer surface thereof toprotect the optical fiber during connectorization.

BACKGROUND OF THE ART

In the construction of glass-based optical fiber elements, a coating isusually applied to the glass optical fiber immediately after drawing toprotect the glass surface from the detrimental effects of chemicaland/or mechanical attack which would otherwise occur. Such forms ofattack, to which glass optical fibers are particularly susceptible,greatly decrease the mechanical strength of optical fibers and lead totheir premature failure.

Conventionally, several coatings are applied to optical fibers, witheach serving a specific purpose. A soft coating is applied initially toprotect the fiber from microbending losses, and a harder, secondarycoating is applied over the soft coating to provide resistance toabrasion.

The connectorization process (i.e., coupling an optical fiber element toanother optical fiber element or other optical element via a splicingdevice or optical connector) conventionally entails the removal of allcoating layers such that the bare glass surface is exposed. The glasssurface is usually cleaned by wiping it with a soft tissue which hasbeen moistened with an alcohol such as isopropanol. The fiber is thenfixed into a connector ferrule or splicing device using an adhesive suchas an epoxy, hot melt, or acrylic adhesive. Upon curing (or cooling) ofthe adhesive, the fiber end face is polished and the connectorizationprocess is complete.

During the connectorization process, the optical fiber is veryvulnerable. Initially, the fiber may be nicked by the blades of the toolused to remove the outer coatings during the stripping operation. Afterstripping, the bare fiber is exposed to elements in the localenvironment. These are likely to include water vapor and dust particles.Water acts chemically on the surface of the glass and dust acts as anabrasive. Both of these effects contribute to failure of the glassfiber. Most failures in optical fiber systems tend to occur at the sitesof connector installation.

One solution to the problem of fiber stripping and exposure duringconnectorization has been proposed in U.S. Pat. No. 4,973,129. Thatpatent discloses an optical fiber element wherein a resin compositionhaving a Shore D hardness value of 65 or more (specified in the JapaneseIndustrial Standards at room temperature) is applied to the surface of aglass optical fiber having a numerical aperture (NA) value of 0.35 ormore. The resin is then cured to form a primary coating layer which doesnot have to be peeled from the optical fiber at the time ofconnectorization. Instead, the primary layer remains on the fiber duringconnectorization (and thereafter) to prevent the fiber from beingdamaged as described above. Useable optical fibers are said to belimited to those having a NA value of 0.35 or more because the opticallosses caused by microbending ("microbending loss") increase uponcovering the optical fiber with such a hard resin. In optical fiberswith a NA value below 0.35, microbending loss was found to be so greatas to make optical communications impractical. When the NA of theoptical fiber is 0.35 or more, however, microbending loss was not foundto be a problem.

Unfortunately, optical fiber elements which require an optical fiberhaving a NA value of 0.35 or more are not commercially useful. As isknown, NA is a measure of the angle of light which will be accepted andtransmitted in an optical fiber. Optical fiber elements having a NAvalue of 0.35 or more find limited use in communication, datatransmission, and other high bandwidth applications for two reasons: 1)limited information-carrying capacity and 2) incompatibility withexisting, standardized communication fibers (which normally have NAvalues less than 0.29). The information-carrying capacity of an opticalfiber is usually expressed as bandwidth. Bandwidth is a measure of themaximum rate at which information can pass through an optical fiber(usually expressed in MHz-km). Bandwidth is inversely proportional to NAbecause the higher order modes (analogous to higher angles of incidentlight) have longer paths in the fiber, thereby resulting in pulsebroadening or dispersion. The bandwidth limitation of an optical fiberelement occurs when individual pulses travelling through that fiber canno longer be distinguished from one another due to dispersion. Thus, thelarger the NA value of an optical fiber, the smaller is that fiber'sbandwidth (and therefore information-carrying capacity). Mostcommercially useful optical fibers have a NA value of 0.29 or less. Ascompared to the information-carrying capacity of such commerciallyuseful optical fibers, fibers having a NA value of 0.35 or more carryfar less information in a given period of time and are thereforeundesirable.

Incompatibility becomes a problem when one optical fiber element isspliced or connected to another optical fiber element. In this instance,it is important to minimize signal attenuation at the point ofconnection. When an optical fiber element with a higher NA value isspliced to a fiber with a lower NA value, all light exceeding the NAvalue of the receiving fiber will be attenuated. Light-carrying capacityis proportional to the square of the NA. Thus, as an example, 38% of thelight will be lost when transmitted from a fiber with a NA value of 0.35to a fiber with a NA value of 0.275. This is a significant andunacceptable loss in signal.

Accordingly, a need exists in the art for an optical fiber element whichprotects the optical fiber during connectorization and which allows theuse of optical fibers having NA values smaller than 0.35.

SUMMARY OF THE INVENTION

The present invention provides an optical fiber element comprising anoptical fiber having a numerical aperture ranging from 0.08 to 0.34 anda protective coating affixed to the outer surface of the optical fiber.The protective coating has a Shore D hardness value of 65 or more andremains on the optical fiber (i.e., is not stripped from the fiber)during connectorization so that the fiber is neither damaged by theblades of a stripping tool nor subjected to chemical or physical attackby, e.g., water vapor or dust.

It is preferred that the optical fiber element further include a bufferwhich substantially encloses the optical fiber and the protectivecoating. The buffer may include an inner, resilient layer and an outer,rigid layer. The inner, resilient layer is preferably of sufficientlylow modulus (e.g., 0.5 to 20 MPa) to provide the optical fiber elementwith protection against microbending losses. The outer, rigid layer ispreferably of sufficiently high modulus (e.g., 500 to 2500 MPa) toprotect the underlying layers from abrasion and mechanical damage.

The protective coating preferably forms an adhesive bond with both theoptical fiber and with the inner, resilient layer of the buffer. In thismanner, the protective coating and buffer form an integral coating.During connectorization, however, enough of the buffer must be removedto allow the optical fiber and protective coating to be inserted in andadhered to a connector or splicing device. To facilitate this, the bondformed between the protective coating and the optical fiber is greaterthan that formed between the protective coating and the inner, resilientlayer, thereby allowing the buffer to be easily stripped from the fiberand protective coating.

The present invention also provides a method for producing an opticalfiber element. The method comprises the steps of:

providing an optical fiber having a numerical aperture ranging from 0.08to 0.34; and

affixing a protective coating to the outer surface of the optical fiber,the protective coating preferably having a Shore D hardness value of 65or more.

The method may further include the step of applying a buffer whichsubstantially encloses the optical fiber and the protective coating, thebuffer including an inner, resilient layer and an outer, rigid layer.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates an optical fiber element constructed in accordancewith the present invention, including an optical fiber, a protectivecoating, and a buffer;

FIG. 2 graphically illustrates a dynamic fatigue analysis (Weibull plot)for the optical fiber element of Example 1;

FIG. 3 graphically illustrates microbending test results for Examples 2,3, 5, and 6; and

FIG. 4 graphically illustrates macrobending test results for Examples 5and 6.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an optical fiber element 10 constructed in accordance withthe present invention. Optical fiber element 10 includes an opticalfiber 12, a protective coating 14, and a buffer 16. Optical fiber 12further includes a core 12A and cladding 12B. Core 12A and cladding 12Bare preferably constructed of glass, but may also be constructed of anysuitable material. For example, core 12A can also be made frompoly(methyl methacrylate), polystyrene, polycarbonate, alloys of theforegoing, fluorinated or deuterated analogs to the foregoing,fluoropolymers and alloys thereof, and silicones. Cladding 12B can alsobe constructed from materials other than glass, such as fluoropolymers,fluoroelastomers, and silicones. Buffer 16 longitudinally enclosesoptical fiber 12 and protective coating 14, and preferably includes aninner, resilient layer 18 and an outer, rigid layer 20. Inner, resilientlayer 18 provides optical fiber element 10 with protection againstmicrobending losses while outer, rigid layer 20 protects the underlyinglayers from abrasion and mechanical damage.

Optical fiber 12 may have any desired numerical aperture (NA) range, butpreferably has a NA value ranging from 0.08 to 0.34. Further, opticalfiber 12 may be either a single mode fiber (i.e., supports only one paththat a light ray can follow in travelling down the optical fiberelement) or a multi-mode fiber (i.e., capable of supporting multiplepaths for light rays to follow in travelling down the optical fiberelement). When optical fiber 12 is a single mode fiber, the NA thereofpreferably ranges from about 0.11 to about 0.20. When optical fiber 12is a multi-mode fiber, the NA thereof preferably ranges from about 0.26to about 0.29.

Protective coating 14 is affixed to the outer surface of optical fiber12 (or, more precisely, to the outer surface of cladding 12B). Duringthe process of connectorization, buffer 16 is stripped from apredetermined length of a terminal end of optical fiber element 10 toallow the fiber to be properly inserted into and bonded with an opticalfiber connector or splicing device. Protective coating 14, however,remains on the outer surface of optical fiber 12 (i.e., is not strippedfrom the fiber) during the process of connectorization and permanentlythereafter. In this manner, protective coating 14 prevents optical fiber12 from being damaged by the blades of a stripping tool (used to removebuffer 16) or weakened by chemical or physical attack from, e.g., watervapor or dust, which would otherwise occur if the bare glass surface ofoptical fiber 12 were exposed. Protective coating 14 should have asufficiently high degree of hardness that the coating is resistant tomechanical force and abrasion. Specifically, protective coating 14should allow optical fiber element 10 to be handled, stripped, cleaned,and clamped inside of a connector or splicing device without incurringdamage to the surface of optical fiber 12. Further, once clamped andbonded inside of a connector or splicing device, protective coating 14should be hard enough that optical fiber 12 does not exhibit signal lossdue to radial movement of the coated fiber inside of the connector orsplicing device. Protective coating 14 is sufficiently hard for thesepurposes when it has a Shore D hardness value of 65 or more (asdetermined in accordance with ASTM D2240).

In addition to a Shore D hardness value of 65 or more, the idealprotective coating would also provide the following:

1) a barrier to water vapor, dust, and other agents of chemical andmechanical attack against the glass optical fiber;

2) surface characteristics such that the protective coating adheresstrongly to the glass outer surface of the optical fiber so that it isnot easily removed, and at the same time adheres weakly to the buffer sothat the buffer can be easily stripped from the coated optical fiberwithout causing damage to the fiber; and

3) the ability to form strong bonds with the adhesives used to affixoptical fibers to connectors and splicing devices.

Any coating having a Shore D hardness value of 65 or more and whichprovides, at least to some degree, all or most of the above-listedproperties may be utilized as protective coating 14. While the presentoptical fiber element is not limited to any particular group ofprotective coating materials, a number of suitable materials have beenidentified. For example, the protective coating may comprise anepoxy-functional polysiloxane having the structure: ##STR1## wherein:the ratio of a to b ranges from about 1:2 to about 2:1; and

R is an alkyl group of one to three carbon atoms.

Such epoxy-functional polysiloxanes are described in U.S. Pat. No.4,822,687, the disclosure of which is incorporated herein by reference,and in copending U.S. patent application Ser. No. 07/861,647, filed Apr.1, 1992.

Preferably, the protective coating further includes a bisphenol Adiglycidyl ether resin having the structure: ##STR2## wherein theaverage value of n ranges from 0 to 2. More preferably, the averagevalue of n is less than 1.

Suitable bisphenol A diglycidyl ether resins are commercially availablefrom The Dow Chemical Company as D.E.R.™331 and D.E.R.™332, and alsofrom Shell Oil Company as Epon™828. The bisphenol A diglycidyl etherresin may be present in the protective coating at a weight percentageranging from about 0 to 20, and the epoxy-functional polysiloxane may bepresent at a weight percentage ranging from about 80 to 100. The weightpercentage of bisphenol A diglycidyl ether resin in the protectivecoating may be extended to about 30 by decreasing the upper limit of theratio range of a to b in the epoxy-functional polysiloxane to about1.5:1 (so that the range is from 1:2 to 1.5:1). It should be noted thatthe protective coating may contain other constituents (e.g., catalysts,sensitizers, stabilizers, etc.). Thus, the above weight percentages arebased only upon the total amount of epoxy-functional polysiloxane andbisphenol A diglycidyl ether resin present in the protective coating.

As an additional example of a protective coating material, the bisphenolA diglycidyl ether resin set forth above may alone be used as theprotective coating (i.e., without any epoxy-functional polysiloxane).

A further example of a suitable protective coating material includes theabove-described epoxy-functional polysiloxane along with acycloaliphatic epoxide having the structure: ##STR3## Suitablecycloaliphatic epoxides are commercially available from Union Carbideunder the . .tradname.!. .Iadd.trade name .Iaddend.ERL-4221. In thisinstance, the ratio of a to b in the epoxy-functional polysiloxanepreferably ranges from about 1:2 to about 1.5:1, and the cycloaliphaticepoxide is present in the protective coating at a weight percentageranging from about 0 to 50 (with the balance comprising epoxy-functionalpolysiloxane). As before, the weight percentages are based on the totalamount of epoxy-functional polysiloxane and cycloaliphatic epoxide inthe protective coating.

Another example of an appropriate protective coating includes theaforementioned epoxy-functional polysiloxane and cycloaliphatic epoxidealong with an alpha-olefin epoxide having the structure: ##STR4##wherein R is an alkyl of 10 to 16 carbon atoms. Such alpha-olefinepoxides are commercially available as Vikolox™ from Atochem NorthAmerica, Inc., Buffalo, N. Y. Preferably, the alpha-olefin epoxide ispresent in the protective coating at a weight percentage of about 20,the cycloaliphatic epoxide is present at a weight percentage rangingfrom about 27 to 53, and the epoxy-functional polysiloxane makes up thebalance. Further, the ratio of a to b in the epoxy-functionalpolysiloxane desirably ranges from about 1.5:1 to 2:1. Again, the weightpercentages are based on the total amount of epoxy-functionalpolysiloxane, cycloaliphatic epoxide, and alpha-olefin epoxide in theprotective coating.

A further example of a protective coating according to the presentinvention is a novolac epoxy having the structure: ##STR5## wherein theaverage value of n ranges from 0.2 to 1.8. The preferred value is 0.2.Such novolac epoxies are commercially available from The Dow ChemicalCompany as D.E.N.™431, D.E.N.™438, and D.E.N.™439.

As noted above, buffer 16 preferably includes an inner, resilient layer18 and an outer, rigid layer 20. It has been found that by including arelatively soft, resilient layer (18) to the outer, longitudinal surfaceof protective coating 14, microbending losses are minimized. Thus, eventhough protective coating 14 has a high degree of hardness (Shore Dhardness of 65 or more), the relatively soft inner, resilient layer 18allows optical fibers having virtually any NA value to be used in thepresent optical fiber element without incurring unacceptably highmicrobending losses. For this reason, commercially useful optical fibershaving NA values ranging from 0.08 to 0.34 may be used.

In order to provide sufficient protection from microbending losses,inner, resilient layer 18 preferably has a modulus ranging from 0.5 to20 MPa. It is also preferred that inner, resilient layer 18 be capableof bonding with protective coating 14. In this manner, protectivecoating 14 and buffer 16 together form an integral coating for opticalfiber 12. However, the bond between protective coating 14 and inner,resilient layer 18 should be sufficiently weak that buffer 16 can beeasily stripped from protective coating 14. Specifically, the bondbetween protective coating 14 and optical fiber 12 should be strongerthan the bond between protective coating 14 and inner, resilient layer18. In this manner, buffer 16 can be readily stripped from optical fiberelement 10 without also removing protective coating 14 or causing damageto optical fiber 12.

Inner, resilient layer 18 may be constructed from any material havingthe foregoing physical properties. Examples of suitable materialsinclude acrylate or epoxy functional urethanes, silicones, acrylates,and epoxies. Materials which are easily cured using ultravioletradiation are preferred. Such materials are commercially availablewithin the desired modulus range of 0.5 to 20 MPa. Acrylate functionalsilicones, such as those which are commercially available from Shin-EtsuSilicones of America, Inc., Torrance, Calif., are preferred. Aparticularly preferred acrylate functional silicone is Shin-Etsu OF-206,which was determined to have a modulus of 2.5 MPa at room temperature.

Outer, rigid layer 20 protects the underlying coatings from abrasion andcompressive forces. To this end, it is preferred that outer, rigid layer20 has a modulus ranging from 500 to 2500 MPa. Non-limiting examples ofacceptable materials from which outer, rigid layer 20 may be constructedinclude acrylate or epoxy functional urethanes, silicones, acrylates,and epoxies. Acrylate functional urethanes are preferred. Such acrylatedurethanes are commercially available from DSM Desotech, Inc., Elgin,Ill. A particularly preferred acrylated urethane, having a modulus of1300 MPa (23° C.), is available from DeSotech, Inc. as DeSolite®950-103.

The diameters of optical fiber element 10 and optical fiber 12, as wellas the thicknesses of protective coating 14 and buffer 16, will varydepending upon the particular application in which the optical fiberelement is used. Generally, it is preferred that the combined diameterD_(o) of optical fiber 12 and protective coating 14 be compatible withthe connector, splicing device, or other optical element into which thecoated optical fiber is to be inserted. Thus, the diameter D_(o) shouldbe no larger (nor much smaller) than that which can be accommodated bysuch elements. In this regard, it has been found that when the diameterD_(o) ranges from about 120 to 130 micrometers, and is preferably about125 micrometers, optical fiber element 10 will be compatible with mostcommercially available connectors, splicing devices, and other opticalelements. At such a diameter, protective coating 14 may range inthickness from about 8 to about 23 micrometers, cladding 12B may rangein thickness from about 8 to about 24 micrometers, and core 12A willgenerally be about 62.5 micrometers in diameter. It should beunderstood, however, that such thicknesses/diameters are merelyrepresentative of current industry standards, and may be changed withoutdeviating from the scope of the present invention.

In further accordance with current industry standards, the totaldiameter of optical fiber element 10 preferably ranges from about 240 toabout 260 micrometers. As such, the thickness of inner, resilient layer18 preferably ranges from about 15 to about 38 micrometers, and thethickness of outer, rigid layer 20 preferably ranges from about 25 toabout 48 micrometers. Again, such dimensions are merely representativeof current industry standards. The scope of the present invention is notlimited to any particular set of thicknesses or diameters.

Optical fiber element 10 may be produced by any conventional opticalfiber production technique. Such techniques generally involve a drawtower in which a preformed glass rod is heated to produce a thin fiberof glass. The fiber is pulled vertically through the draw tower. Alongthe way, the fiber passes through one or more coating stations in whichvarious coatings are applied and cured in-line to the newly drawn fiber.The coating stations each contain a die having an exit orifice which issized to apply the desired thickness of the particular coating to thefiber. Concentricity monitors and laser measuring devices are providednear each coating station to ensure that the coating applied at thatstation is coated concentrically and to the desired diameter.

To facilitate the coating process, the compositions giving rise toprotective coating 14 and buffer 16 preferably have a viscosity rangingfrom 800 to 15,000 cps, and more preferably from 900 to 10,000 cps.Conveniently, inner, resilient layer 18 and outer, rigid layer 20 can bewet coated in the same coating station and then cured simultaneously.

In order that the invention may be more readily understood, reference ismade to the following examples, which are intended to be illustrative ofthe invention, but are not intended to be limiting in scope.

FIBER DRAWING PROCESS Preform Preparation

A fiber optic preform was first prepared in accordance with U.S. Pat.No. 4,217,027.

Fiber Drawing

The fiber optic draw tower used in the draw process was based on anenclosed Nokia system which featured a Nokia-Maillefer fiber draw tower(Vantaa, Finland). To begin the draw process, a downfeed system was usedto control the rate at which the optical preform was fed into a 15 KWLapel zirconia induction furnace (Lapel Corp., Maspeth, NY) in which thepreform was heated to a temperature at which it may be drawn to fiber(between about 2200° to 2250° C.). Below the heat source, a LaserMike™laser telemetric measurement system was used to measure the drawn fiberdiameter as well as monitor the fiber position within the tower.

The newly formed fiber was then passed to a primary coating station atwhich the protective coating was applied. The coating station included acoating die assembly, a Fusion Systems® Corp. microwave UV curingsystem, a concentricity monitor, and another laser telemetric system.The coating die assembly, based on a Norrsken Corp. design, consisted ofa sizing die(s), back pressure die and a containment housing which wasmounted on a stage having adjustment for pitch and tilt and x-ytranslation. These adjustments were used to control coatingconcentricity. The protective coating material was supplied to thecoating die assembly from a pressurized vessel and was applied, curedand measured within the primary coating station.

The coated fiber then proceeded on to a secondary coating station wherea buffer was applied to the coated fiber. In certain cases it wasdesirable to apply two buffer layers simultaneously in a wet-on-wetapplication at the secondary coating station. In this case an additionalsizing die was used and an additional vessel was used to supply materialto this die. The coatings were applied, one after the other, and thencured and the outer diameter measured. As required, additional coatingscould be applied via additional coating stations. Ultimately, thecompleted optical fiber element was drawn through a control capstan andonto a take-up spool (Nokia).

TESTING Coating Dimension

Coating dimensions and concentricities are measured using an OlympusSTM-MJS Measuring Microscope and MeasureGraph 123 software (RoseTechnologies). The technique fits a circle to a number of pointsselected about the circumference. The size of these circles and theiroffset from center (from various components of the fiber structure) weredetermined and reported by the software.

Connector temperature Cycle

This test was modeled after Bellcore test TR-NWT-0003236 (Jun. 1992),"Generic Requirements for Optical Fiber Connectors". The Bellcore testcycles from -45° to 70° C. for 14 days. The test procedure used hereinspanned -45° to 60° C. for 48 hours. The values reported are the maximumwithin this time.

Dynamic Fatigue Testing

This test was performed similarly to Fiber Optic Test Procedure ("FOTP")28. The exceptions are as follows:

Strain Rate =9% minute

Gauge Length =4 meters

Environment =Ambient Laboratory

Microbending Testing

Microbending testing was done in accord with FOTP-68. The highest valueobtained was reported.

Macrobending Testing

Macrobending testing consisted of determining the transmission of afiber that was turned 180° about mandrels of various diameter. Thetransmission was determined as the ratio of the power out of the wrappedfiber/power out of unwrapped fiber. Care was taken to insure that otherloops in the fiber were large enough (radius>10 cm) such that they didnot contribute to the loss.

Numerical Aperture Testing

The numerical attenuation was determined using a Photon Kinetics ModelFOA-2000 which refers to FOTP-177 for "Numerical Aperture Measurement ofGraded-Index Optical Fibers". The test procedure was modified toaccommodate experimental fiber by using shorter lengths of fiber(0.2-0.5 Km) rather than the ≧1 Km lengths specified in the FOTP.

Pull-Out Test

A tensile pull-out test was utilized to determine how well the connectoradhesive adheres to the protective coating (which remains on the fiberduring connectorization and permanently thereafter). An "ST" connectordesign was chosen due to its availability and compatibility with thetest equipment. It consists of a zirconia ferrule mounted in a barrel towhich was attached a bayonet assembly.

Fiber Preparation

In all of the following examples, 12 inch pieces

of the completed optical fiber element were stripped to reveal 1.5-2.0cm of the protective coating which was then cleaned with a tissuemoistened with isopropyl alcohol. The fiber ends were allowed to dryprior to installing connectors.

Two-. .Pail.!. .Iadd.Part .Iaddend.Epoxy

A standard two-part epoxy for fiber optic connectors (either Tra-Con#BA-F112 or 3M #8690, Part No. 80-6107-4207-6) was used. It was mixedaccording to the manufacturer's instructions and poured from the mixingenvelope into a syringe body. A plunger was installed taking care toavoid incorporation of air into the liquid. The syringe was fitted witha blunt-end needle. This assembly was used to inject adhesive into theferrule from the barrel end until adhesive appeared at the tip end. Thefiber was inserted such that the buffer coating bottomed in the barrel.The adhesive was cured for 25 minutes at 90° C.

Hot Melt Adhesive

A polyamide hot melt adhesive was provided preinstalled in ST connectorsas a product (3M 6100 Hot-Melt™ Connector, Part No. 80-6106-2549-5). Theconnector was heated in the required oven (3M Part No. 78-8073-7401-8)for two minutes and removed. The optical fiber was immediately installedsuch that the buffer coating bottomed in the barrel. The connector wasthen left undisturbed until cool.

Pull-Out Testing

Pull-out testing was performed using an Instron tensile tester (Model4201). Peak loads (before pull-out) were recorded. The average of fiveor ten tests was reported as the pull-out value for each sample.

Spectral Attenuation

The spectral attenuation of the fiber was determined using a PhotonKinetics Model FOA-2000. The operational reference was FOTP-46.

Catalyst Formulation

For each of the examples, the following catalyst formulation was used:

40 parts of bis(dodecylphenyl)iodonium hexafluoroantimonate

60 parts of a C10-C14 alcohol blend

4 parts 2-isopropylthioxanthone

EXAMPLES Example 1 Epoxy-Functional Polysiloxane

Protective Coating Formulation

A protective coating formulation was prepared by thoroughly mixing 95parts of an epoxy-functional polysiloxane with 5 parts of theabove-described catalyst formulation. The epoxy-functional polysiloxanehad the structure set forth above in which the ratio of a to b was 1:1and R was a methyl group. This formulation was then filtered though a0.2μpolysulfone filter disk into a amber glass bottle. 1 part of3-glycidoxypropyltrimethoxysilane was added and thoroughly mixed.

Coating Process

The protective coating formulation was coated to a diameter of 125 μm ona 110 μm optical fiber which was freshly drawn from a Diasil™ preform ata draw speed of 30 MPM (meters per minute). The coating was cured and asubsequent layer of an acrylated urethane (Desotech 950-103) buffer wascoated and cured to a diameter of 250 μm.

Dynamic Fatigue Analysis

The completed optical fiber element was subjected to tensile testing tofailure (dynamic fatigue analysis). The Weibull statistics for suchtesting are shown in FIG. 2.

Pull-Out Test

The buffer coatings on this and similarly coated fiber elements wereeasily removed using conventional stripping tools. Connector pull-outtesting gave the following results:

    ______________________________________    Hot melt Adhesive                     5.2 lbs    Two-Part Epoxy   6.1 lbs    ______________________________________

Example 2 Epoxy-Functional Polysiloxane/Bisphenol A: Dual Coat

Protective Coating Formulation

A mixture of 75 parts of the epoxy-functional polysiloxane used inexample 1 was mixed with 25 parts Epon™828 bisphenol A diglycidyl etherresin (from the Shell Oil Co.). 5.3 parts of the catalyst formulationwas added and thoroughly mixed and filtered though a 1.0 μm Teflon™filter disc into an amber glass bottle.

Coating Process

This formulation was coated and cured to a 125 μm diameter on a 100 μmglass fiber which was freshly drawn from a graded index preform at adraw speed of 45 MPM. A buffer coating of acrylated urethane (DSM 950-103 from DSM Desotech, Inc.), having a modulus of 1300 MPa was coatedand cured to a 250 μm diameter.

Microbending Test

The completed optical fiber element was tested for microbendingaccording to FOTP-68 resulting in a maximum loss of 4.4 dB (see FIG. 3).

Pull-Out Test

A similarly coated optical fiber element gave the following results forconnector pull-out tests:

    ______________________________________    Hot melt Adhesive                     7.2 lbs    Two-Part Epoxy   4.4 lbs    ______________________________________

Example 3 Epoxy-Functional Polysiloxane/Bisphenol A; Triple Coat

Protective Coating Formulation

The protective coating formulation was that described in Example 2.

Coating Process

The material was coated and cured to a 125 μm diameter on a 100 μm glassfiber which was freshly drawn from a graded index preform at a drawspeed of 45 MPM. Inner and outer buffer layers (DSM 950-075 and DSM950-103, respectively) were applied then cured simultaneously todiameters of 183 and 226 μm, respectively. The inner buffer layer had amodulus of 3.8 MPa while the outer buffer layer had a modulus of 1300MPa.

Pull-Out Test

The optical fiber element gave the following results for connectorpull-out tests:

    ______________________________________    Hot melt Adhesive                     2.6 lbs    Two-Part Epoxy   6.2 lbs    ______________________________________

Microbending Test

The optical fiber element was tested for microbending according toFOTP-68 resulting in a maximum loss of 1.15 dB (see FIG, 3).

Example 4 Novolac

Protective Coating Formulation

A protective coating formulation was prepared by thoroughly mixing 95parts of an epoxy-novolac (Dow DEN 431) with 5 parts of catalystformulation, This formulation was filtered to 0.5 μm through a Teflon™filter disk into an amber bottle.

Coating Press

The protective coating formulation was coated and cured to a 125 μmdiameter on a 100 μm glass fiber which was freshly drawn from anunpolished preform at a draw speed of 45 MPM, A buffer coating ofacrylated urethane (DSM 9-17) was coated and cured to a 250 μm diameter.

Pull-Out Test

The optical fiber element gave the following results for connectorpull-out tests:

    ______________________________________    Hot melt Adhesive                     6.2 lbs    Two-Part Epoxy   6.5 lbs    ______________________________________

Example 5 Epoxy-Functional Polysiloxane/Bisphenol A; Triple Coat

Protective Coating Formulation

A mixture of 75 parts of the epoxy-functional polysiloxane used inexample 1 was mixed with 25 parts Epon™828 bisphenol A diglycidyl etherresin (from the Shell Oil Co.). 10 parts of the catalyst formulation wasadded and the formulation was thoroughly mixed and filtered though a 1.0μm Teflon™ filter disc into an amber glass bottle.

Coating Process

The protective coating formulation was coated and cured to a 125 μmdiameter on a 100 μm glass fiber which was freshly drawn from a gradedindex preform at a draw speed of 45 MPM. Inner and outer buffer layers(Shin-Etsu OF 206 and DSM 950-103, respectively) were applied and thencured simultaneously to diameters of 184 and 250 μm, respectively. Theinner buffer layer had a modulus of 2.5 MPa while the outer buffer layerhad a modulus of 1300 MPa.

Pull-Out Test

The optical fiber element gave the following results for connectorpull-out tests:

    ______________________________________    Hot melt Adhesive                     3.0 lbs    Two-Part Epoxy   6.4 lbs    ______________________________________

Microbending Test

The optical fiber element was tested for microbending according toFOTP-68 resulting in a maximum loss of 0.76 dB (see FIG. 3).

Macrobending Test

The fiber was tested for macrobending and the results are shown in FIG.4.

Numerical Aperture

The numerical aperture was determined to be 0.258

Spectral Attenuation

The spectral attenuation, based on the modified FOTP-46, was determinedto be as follows:

@850 nm=6.03 db/Km

@1300 nm=3.5 db/Km

(Comparative) Example 6 Corning 62.5/125 μm

The fiber used for this comparative example was Corning™ Optical Fiberwith the following identifications:

    ______________________________________    Product:     LNF(™)     62.5/125 Fiber    Coat:        CPC3    Fiber ID:    262712272304    ______________________________________

Pull-Out Test

The fiber gave the following results for connector pull-out tests:

    ______________________________________    Hot melt Adhesive                     5.9 lbs    Two-Part Epoxy   4.6 lbs    ______________________________________

Microbending Test

The fiber was tested for microbending according to FOTP-68 resulting ina maximum loss of 0.42 dB (see FIG. 3 ).

Macrobending Test

The fiber was tested for macrobending and the results are shown in FIG.4.

Numerical Aperture A value for numerical aperture of 0.269 was providedby Corning (method unspecified).

Spectral Attenuation

The spectral attenuation values provided by Corning (method unspecified)were as follows:

@850 nm =2.7 db/. .Km.!. .Iadd.km .Iaddend.

@1300 nm =0.6 db/. .Kin.!. .Iadd.km .Iaddend.

Example 7 Hardness Testing

Shore D hardness values of various protective coating formulations wereevaluated using a Shore D durometer mounted on Shore Leverloaderfollowing the general procedure of ASTM D-2240. The samples wereprepared by curing thin layers on top of each other such that largediscs of the material resulted. Samples were at room temperature (23°C.) for testing.

    ______________________________________    Formulation 1    95%      Epoxy functional polysiloxane in which             the a:b ratio is 1:1 and R is a methyl             group; and             Catalyst    Formulation 2    71.2%    Epoxy-functional polysiloxane in which             the a:b ratio is 1:1 and R is a methyl             group;    23.8%    Epon ™ 828 bisphenol A  piglycidyl! diglycidylether             resin; and             Catalyst    Formulation 3    31.7%    Epoxy-functional polysiloxane in which             the a:b ratio is 2:1 and R is a methyl             group;    63.3%    ERL-4221 cycloaliphatic epoxide; and             Catalyst    Formulation 4    95%      Dow D.E.N. ™ 431 novolac epoxy; and     5%      Catalyst    ______________________________________    RESULTS: Shore D Hardness    Formulation              No. of Tests                          Average Value                                     Deviation    ______________________________________    1         *           *          *    2         6           70         2    3         6           71         3    4         7           77         4    ______________________________________     *due to brittleness of the sample it fractured upon penetration of the     durometer point; therefore, accurate values were unattainable.

What is claimed is:
 1. An optical fiber element, comprising:an opticalfiber having a numerical aperture ranging from 0.08 to 0.34; . .and.!. aprotective coating affixed to the outer surface of said optical fiber,said protective coating having a Shore D hardness value of 65 ormore.Iadd., wherein the protective coating remains on the outer surfaceof the optical fiber during connectorization and permanentlythereafter.Iaddend..
 2. The optical fiber element of claim 1 furtherincluding a buffer which substantially encloses said optical fiber andsaid protective coating, said buffer comprising an inner, resilientlayer and an outer, rigid layer.
 3. The optical fiber element of claim 2wherein said inner, resilient layer has a modulus ranging from 0.5 to 20MPa, and said outer, rigid layer has a modulus ranging from 500 to 2500MPa.
 4. The optical fiber element of claim 2 wherein said inner,resilient layer has a thickness ranging from 15 to 38 micrometers, andsaid outer, rigid layer has a thickness ranging from 25 to 48micrometers.
 5. The optical fiber element of claim 2 wherein saidprotective coating adhesively bonds with said optical fiber and withsaid inner, resilient layer, said bond with said optical fiber beingstronger than said bond with said inner, resilient layer.Iadd., wherebythe buffer may be stripped from the protective coating duringconnectorization such that the protective coating remains on the outersurface of the optical fiber during connectorization and permanentlythereafter.Iaddend..
 6. The optical fiber element of claim 1 whereinsaid protective coating comprises an epoxy-functional polysiloxanehaving the structure: ##STR6## wherein: the ratio of a to b ranges fromabout 1:2 to about 2:1; andR is an alkyl group of one to three carbonatoms.
 7. The optical fiber element of claim 6 wherein said protectivecoating further comprises a bisphenol A diglycidyl ether resin havingthe structure: ##STR7## wherein n ranges from 0 to
 2. 8. The opticalfiber element of claim 7 wherein said bisphenol A diglycidyl ether resinis present in said protective coating at a weight percentage rangingfrom about 0 to about 20, said weight percentage based on the totalamount of epoxy-functional polysiloxane and bisphenol A diglycidyl etherresin present in said protective coating.
 9. The optical fiber elementof claim 7 wherein the ratio of a to b ranges from about 1:2 to about1.5:1, and wherein said bisphenol A diglycidyl ether resin is present insaid protective coating at a weight percentage ranging from about 0 toabout 30, said weight percentage based on the total amount ofepoxy-functional polysiloxane and bisphenol A diglycidyl ether resinpresent in said protective coating.
 10. The optical fiber element ofclaim 6 wherein said protective coating further comprises acycloaliphatic epoxide having the structure: ##STR8##
 11. The opticalfiber element of claim 10 wherein the ratio of a to b ranges from about1:2 to about 1.5:1, and wherein said cycloaliphatic epoxide is presentin said protective coating at a weight percentage ranging from about 0to about 50, said weight percentage based on the total amount ofepoxy-functional polysiloxane and cycloaliphatic epoxide present in saidprotective coating.
 12. The optical fiber element of claim 10 whereinsaid protective coating further includes an alpha-olefin epoxide havingthe structure: ##STR9## wherein R is an alkyl of 10 to 16 carbon atoms.13. The optical fiber element of claim 12 wherein:the ratio of a to branges from about 1.5:1 to about 2:1; said epoxy-functional polysiloxaneis present in said protective coating at a weight percentage rangingfrom about 27 to about 53; said cycloaliphatic epoxide is present insaid protective coating at a weight percentage ranging from about 27 toabout 53; and said alpha-olefin epoxide is present in said protectivecoating at a weight percentage of about 20, said weight percentagesbased on the total amount of epoxy-functional polysiloxane,cycloaliphatic epoxide, and alpha-olefin epoxide present in saidprotective coating.
 14. The optical fiber element of claim 1 whereinsaid protective coating comprises a novolac epoxy having the structure:##STR10## wherein the average value of n ranges from 0.2 to 1.8.
 15. Theoptical fiber element of claim 1 wherein said protective coatingcomprises a bisphenol A diglycidyl ether resin having the structure:##STR11## wherein n ranges from 0 to
 2. 16. The optical fiber element ofclaim 1 wherein said protective coating has a thickness ranging from 8to 23 micrometers.
 17. The optical fiber element of claim 2 wherein saidoptical fiber and said protective coating have a combined diameterranging from about 120 to about 130 micrometers.
 18. The optical fiberelement of claim 17 wherein the total diameter of said optical fiberelement ranges from about 240 to about 260 micrometers.
 19. The opticalfiber element of claim 1 wherein said optical fiber is capable ofsupporting multiple modes and has a numerical aperture ranging fromabout 0.26 to about 0.29.
 20. The optical fiber element of claim 1wherein said optical fiber is capable of supporting one mode and has anumerical aperture ranging from about 0.11 to about 0.20.
 21. A methodfor producing an optical fiber element comprising the steps of:providingan optical fiber having a numerical aperture ranging from 0.08 to 0.34;and affixing a protective coating to the outer surface of said opticalfiber, said protective coating having a Shore D hardness value of 65 ormore.
 22. The method of claim 21 further including the step of applyinga buffer which substantially encloses said optical fiber and saidprotective coating, said buffer comprising an inner, resilient layer andan outer, rigid layer.
 23. The method of claim 22 wherein said inner,resilient layer has a modulus ranging from 0.5 to 20 MPa, and saidouter, rigid layer has a modulus ranging from 500 to 2500 MPa.
 24. Themethod of claim 23 wherein said inner, resilient layer is applied at athickness ranging from 15 to 38 micrometers, and said outer, rigid layeris applied at a thickness ranging from 25 to 48 micrometers.
 25. Themethod of claim 22 wherein said protective coating adhesively bonds withsaid optical fiber and with said inner, resilient layer, said bond withsaid optical fiber being stronger than said bond with said inner,resilient layer.
 26. The method of claim 22 wherein said optical fiberand said protective coating have a combined diameter ranging from about120 to about 130 micrometers.
 27. The method of claim 26 wherein thetotal diameter of said optical fiber element ranges from about 240 toabout 260 micrometers.
 28. The method of claim 21 wherein saidprotective coating is applied at a thickness ranging from 8 to 23micrometers.
 29. The method of claim 21 wherein said protective coatingcomprises an epoxy-functional polysiloxane having the structure:##STR12## wherein: the ratio of a to b ranges from about 1:2 to about2:1; andR is an alkyl group of one to three carbon atoms.
 30. The methodof claim 29 wherein said protective coating further comprises abisphenol A diglycidyl ether resin having the structure: ##STR13##wherein n ranges from 0 to
 2. 31. The method of claim 30 wherein saidbisphenol A diglycidyl ether resin is present in said protective coatingat a weight percentage ranging from about 0 to about 20, said weightpercentage based on the total amount of epoxy-functional polysiloxaneand bisphenol A diglycidyl ether resin present in said protectivecoating.
 32. The method of claim 30 wherein the ratio of a to b rangesfrom about 1:2 to about 1.5:1, and wherein said bisphenol A diglycidylether resin is present in said protective coating at a weight percentageranging from about 0 to about 30, said weight percentage based on thetotal amount of epoxy-functional polysiloxane and bisphenol A diglycidylether resin present in said protective coating.
 33. The method of claim29 wherein said protective coating further comprises a cycloaliphaticepoxide having the structure: ##STR14##
 34. The method of claim 33wherein the ratio of a to b ranges from about 1:2 to about 1.5:1, andwherein said cycloaliphatic epoxide is present in said protectivecoating at a weight percentage ranging from about 0 to about 50, saidweight percentage based on the total amount of epoxy-functionalpolysiloxane and cycloaliphatic epoxide present in said protectivecoating.
 35. The method of claim 33 wherein said protective coatingfurther includes an alpha-olefin epoxide having the structure ##STR15##wherein R is an alkyl of 10 to 16 carbon atoms.
 36. The method of claim35 wherein:the ratio of a to b ranges from about 1.5:1 to about 2:1;said epoxy-functional polysiloxane is present in said protective coatingat a weight percentage ranging from about 27 to about 53; saidcycloaliphatic epoxide is present in said protective coating at a weightpercentage ranging from about 27 to about 53; and said alpha-olefinepoxide is present in said protective coating at a weight percentage ofabout 20, said weight percentages based on the total amount ofepoxy-functional polysiloxane, cycloaliphatic epoxide, and alpha-olefinepoxide in said protective coating.
 37. The method of claim 21 whereinsaid protective coating comprises a novolac epoxy having the structure:##STR16## wherein n ranges from 0.2 to 1.8.
 38. The method of claim 21wherein said protective coating comprises a bisphenol A diglycidyl etherresin having the structure: ##STR17## wherein n ranges from 0 to 2..Iadd.
 39. A method for connecting an optical fiber element to a device,wherein the optical fiber element comprises:an optical fiber with anumerical aperture ranging from 0.08 to 0.34; a protective coatingaffixed to the outer surface of said optical fiber, said protectivecoating having a Shore D hardness value of 65 or more; and a bufferwhich substantially encloses said optical fiber and said protectivecoating;the method comprising: removing the buffer from the protectivecoating such that the protective coating remains affixed to the outersurface of the optical fiber; and inserting the optical fiber withaffixed protective coating into the device to provide opticalinterconnection..Iaddend.